LCDs have been widely used as optical display devices since Liquid Crystal Displays (LCDs) have low power consumption, a small volume and are light, such that they can be easily carried, as compared to cathode ray tube displays. In general, LCDs have a basic configuration in which polarizing plates are installed on both sides of liquid crystal cells, and the alignment of liquid crystal cells changes according as if an electric field has been applied to the driving circuit. Accordingly, visualization of light is accomplished as characteristics of light transmitted through the polarizing plates vary. Path and birefringent properties of light vary according to an angle of incident light, since liquid crystal is an anisotropic substance having two different refractive indexes.
Due to such properties, LCDs may be problematic, in that LCDs have varied contrast ratios as measures for estimating how clearly images are shown according to viewing angles, and gray scale inversion phenomena may be generated in LCDs to thereby result in low visibility.
In order to overcome such problems, optical compensation films compensating optical retardations generated in liquid crystals are used in liquid crystal display devices, and such optical compensation films may include stretched birefringent polymeric films.
Examples of materials for stretched birefringent polymeric films may include polymethyl methacrylate (PMMA), polystyrene (PS), polycarbonate (PC), a maleimide copolymer, and cyclic polyolefin. The polycarbonate (PC), the maleimide copolymer, and the cyclic polyolefin, among the examples, are optically anisotropic polymeric materials which increase refractive indexes in the alignment direction when molecular chains of the materials are stretched and aligned, i.e., have positive birefringence properties. On the other hand, PMMA or PS is an optically anisotropic polymeric material which increases the refractive index in a direction that is different from the alignment direction when molecular chains of the material are stretched and aligned, i.e., has negative birefringent properties. Examples of polymeric materials mainly used at present in optical compensation films for improving viewing angles of liquid crystal displays may include polycarbonate, maleimide copolymer, and cyclic polyolefin.
On the other hand, various liquid crystal modes have been developed to secure vivid image qualities and wide optical viewing angles in the LCDs, and typically include Double Domain TN (Twisted Nematic), ASM (axially symmetric aligned microcell), OCB (optically compensated blend), VA (vertical alignment), MVA (multidomain VA), SE (surrounding electrode), PVA (patterned VA), IPS (in-plane switching), and FFS (fringe-field switching) modes. These respective modes have inherently aligned liquid crystals and innate optical anisotropies. Therefore, films compensating optical anisotropies corresponding to the respective modes are required to compensate for retardations due to optical anisotropies of these liquid crystal modes. Particularly in case of the IPS mode, liquid crystals having positive dielectric constant anisotropies are filled between polarizing plates. Therefore, the liquid crystals are aligned such that refractive indexes of the planar direction are larger than those of the thickness direction.
Therefore, studies of anisotropic films that can be used as optical compensation films of the IPS mode have been undertaken. Results of the studies, vertically aligned liquid crystal films, and biaxially stretched polymeric films having negative birefringent properties such as polycarbonate and polymethyl methacrylate have been suggested.
However, there are problems in that costs for the coating process are generated, there is a relatively large non-uniformity in retardations even by a minute difference between coating thickness values, and optical defects occur due to foreign objects such as dust may remain on the surface of a coating substrate film or may be present in the liquid crystal solution since vertically aligned liquid crystal films are prepared by accurately coating a bar-shaped low or high molecular weight liquid crystal molecule to a thickness of several microns (μm) on a transparent substrate. In case of a biaxially stretched film of a polymer such as polycarbonate or polymethyl methacrylate, having negative birefringent properties, there is a problem in that heat resistance may be insufficient due to a glass transition temperature near 100° C. although there are no problems in the above-mentioned vertically aligned liquid crystal compensation films.